1. Field of the Invention
The present invention relates generally to a single crystal puller for melting a polycrystalline material and pulling a seed crystal from the melt of the polycrystalline material to produce a single crystal, and more particularly to a position adjusting apparatus for adjusting an initial vertical position of the surface of a melt when a crystal growth is started in the single crystal puller.
2. Description of the Related Art
Conventionally, a Czochralski method (hereinafter referred to as the "CZ method") is known as a method of producing a single crystal from a semiconductor polycrystalline material such as silicon. In the CZ method, a highly pure polycrystalline material is melt in a crucible or the like, and a seed crystal is brought into contact with the melt of the material and then pulled upwardly to produce a single crystal.
During the growth of a single crystal, an industrial television camera or the like, installed above the surface of the melt, is generally used to measure an apparent diameter d of a silicon single crystal on the interface between the single crystal and the melt. Then, the pulling rate is controlled to bring an actual diameter D to a predetermined value based on the apparent diameter d. In addition, adjustments of temperature and so on are performed to produce a silicon single crystal ingot substantially in a cylindrical shape.
The actual diameter D of a single crystal may be approximated by the following equation: EQU D=(.alpha.+.beta.X+.gamma.X.sup.2).times.d
where .alpha., .beta., and .gamma. and are predetermined constants, and X is expressed by L-L.sub.0, where L is the distance between the position of an industrial television camera and the surface of a melt, and L.sub.0 is a reference value for L. It should be noted that X/L is a very small number. Therefore, a change in the reference value L.sub.0 causes a deviation in the relationship between the apparent diameter d measured by the industrial television camera and the actual diameter D, so that the diameter of a single crystal cannot be maintained at a predetermined value.
As a result, if a pulled single crystal has an actual diameter larger than a predetermined value, the outer periphery of the single crystal should be ground until the diameter of the single crystal is equal to the predetermined diameter value. Since it is a recent tendency to produce single crystals of larger diameters, the outer peripheral portion lost by the grinding has become an innegligible problem in view of a manufacturing cost.
Since the reference value L.sub.0 for L is typically measured when a seed crystal is in contact with the surface of a melt at the beginning of a single crystal pulling operation, it is a critical problem to adjust the initial position of the melt surface as accurately as possible for the measurement. Thus, apparatuses for adjusting an initial position of the surface of a melt have been proposed for this purpose, and those disclosed in Japanese Patent Publication No. 5-59876 and so on are known.
Recently, it has been found, in single crystal pullers according to the CZ method which use a stranded wire as a suspender for a seed crystal in such a manner that the stranded wire is wound around a drum and unwound therefrom to raise and lower the seed crystal, that as a single crystal grows and becomes heavier, a tension is applied to the wire, which returns the wire in a direction in which the "strand" is loosened, and consequently the wire is extended. For this reason, when a single crystal is pulled upwardly, the wire is wound around a drum in an extended state, with the loosen "strand". However, when a new seed crystal is attached to the suspender after a produced single crystal ingot is removed, and the drum is rotated in the reverse direction to lower the seed crystal, no tension is applied to the wire to cause the strand to restore, whereby the wire become shorter than when it is wound around the drum. It has been found, as a result, that the wire extends approximately 10.0 millimeters per meter when a single crystal reaches a weight of 15 Kg, as can be seen from FIG. 1, thus producing a relatively large error. Therefore, when a moving distance of a single crystal is calculated from a rotating amount of the drum to determine an initial position of the surface of a melt based on this calculation, the extension and contraction of the wire will cause a large error in determining the initial position of the melt surface.
In addition, it has been found, in the above-mentioned single crystal puller according to the CZ method using a wire as a suspender for a seed crystal, that as the wire is left above a melt at a high temperature, the high temperature causes the wire to extend. It has been also found that the extension increases as the wire is left for a longer time. Specifically, as can be seen from FIG. 2, the wire extends 1.2 millimeters per three meters in ten minutes, and thereafter the wire does not extend any more even if it is left in the same situation. Thus, also in this case, when a moving distance of a single crystal is calculated from a rotating amount of the drum to determine an initial position of the surface of the melt based on this calculation, the extension and contraction of the wire will cause an error in determining the initial position of the melt surface.
Further, it has been found, in the above-mentioned single crystal puller according to the CZ method. using a wire as a suspender for a seed crystal, that when a seed crystal is stopped at a reference position, the elevating rate of the seed crystal causes an error between the reference position and the position at which the seed crystal is actually stopped. A conventional single crystal puller is typically provided with a reference position sensor such that a motor for driving a drum is automatically stopped when the reference position sensor detects the tip of a seed crystal. However, a "time delay", though very short, is present from the time the reference position sensor detects the tip of a seed crystal to the time the seed crystal is actually stopped. This time delay causes a deviation from the reference position. Specifically, it has been found that the maximum deviation of the stopping position from the reference position is 0.7 millimeters with the moving rate of a seed crystal at 250 millimeters/minute. Thus, also in this case, when a moving distance of a single crystal is calculated from a rotating amount of the drum to determine an initial position of the surface of a melt based on this calculation, the erroneous stopping position will cause an error in the initial position of the melt surface.
Further, in the above-mentioned single crystal puller according to the CZ method using a wire as a suspender for a seed crystal, after an initial position of the melt surface has been measured, a seed crystal is once raised, and a crucible is then moved in the vertical direction for correctly positioning the crucible at a defined position based on the measured initial position of the surface of the melt. The seed crystal is once raised because if the crucible only were raised without changing the position of the seed crystal, the seed crystal would be submerged in the melt. However, the crucible elevating operation also causes an error in the initial position of the melt surface.
In the conventional methods mentioned above, operations for raising and lowering a seed crystal, vertically moving a crucible, and so on are manually performed by operators. Since the operators carefully manipulate for ensuring a sufficient accuracy, a long time is required for each of the operations. In addition, variations in manipulations depending on individual operators also affect the accuracy.